Method of producing etching mask, etching mask precursor, and oxide layer, and method of manufacturing thin film transistor

ABSTRACT

The etching mask  80  for screen printing according to one embodiment of the present invention includes aliphatic polycarbonate. Further, the method of producing an oxide layer (the channel  44 ) according to one embodiment of the present invention includes: an etching-mask forming step of forming a pattern of the etching mask  80  including aliphatic polycarbonate; a contact step of, after the etching-mask forming step, contacting the oxide layer with a solution for dissolving a portion of the oxide layer (the channel  44 ) which is not protected by the etching mask  80 ; and a heating step of, after the contact step, heating the oxide layer (the channel  44 ) and the etching mask  80  to or above a temperature at which the etching mask  80  is decomposed.

TECHNICAL FIELD

The present invention relates to a method of producing an etching mask,an etching mask precursor, and an oxide layer, and a method of producinga thin film transistor.

BACKGROUND ART

Conventionally, a polycrystalline silicon film or an amorphous siliconfilm has typically been adopted as a channel layer of a thin filmtransistor, which is an example of an electronic device. However, when apolycrystalline silicon film is used, electron scattering at thepolycrystalline grain boundary limits electron mobility, so as to causevariation in transistor property. When an amorphous silicon film isused, electron mobility is very low and an element tends to deterioratewith time, so as to cause extremely low element reliability. In thisregard, an oxide semiconductor has been attracting interests, which ishigher in electron mobility than an amorphous silicon film and has lessvariation in transistor property than a polycrystalline silicon film.Moreover, not only an oxide semiconductor, but also an oxide conductivematerial or an oxide insulator made of an oxide is an essentialtechnical element for realization of an electronic device made of onlyan oxide, for example, and thus is attracting very high industrialinterests.

There has been recently an active attempt to produce an electronicdevice on a flexible resin substrate in accordance with a low-energyproduction process such as a printing method. There is an advantage thata semiconductor layer can be patterned directly on a substrate inaccordance with a printing method or the like and no etching step isthus required for patterning.

For example, Patent Documents 1 to 3 represent attempts for producing acoated flexible electronic device including a conductive polymer or anorganic semiconductor. Further, the present applicant discloses atechnology for solving some of the aforementioned problems (PatentDocument 4).

PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JP PatentPublication (Kokai) No. 2007-134547A Patent Document 2: JP PatentPublication (Kokai) No. 2007-165900A Patent Document 3: JP PatentPublication (Kokai) No. 2007-201056A Patent Document 4: WO2015/019771SUMMARY OF THE INVENTION Problems to be Solved by the Invention

While the industry and consumers demand various information terminalsand information appliances, a semiconductor is required to operate athigher speed, to be stable for a long period of time, and to apply aless environmental load. However, the conventional technique typicallyincludes a vacuum process, a process according to the photolithographytechnique, or the like, which requires a relatively long time periodand/or expensive equipment. Therefore, these processes lead to very lowutilization efficiency of raw materials and production energy. This isnot preferred from the industrial and mass productivity perspectives. Itis very difficult to form layers under current conditions by applyingthe above printing method, such as gravure printing, screen printing,offset printing, or ink jet printing to silicon semiconductors or othersemiconductors that have been mainly used to date. Even in a case whereany one of the conductive polymer and the organic semiconductordisclosed in Patent Documents 1 to 3 is adopted, the electrical propertyand stability thereof remain insufficient. In addition, the term“layer(s)” in this application conceptually includes a layer as well asa film. Similarly, the term “film” in this application conceptuallyincludes a film as well as a layer.

Formation of layers by the above various printing methods, in particularby the screen printing method which is highly applicable to variousdevices such as semiconductor elements and electronic devices, is nowattracting much attention in the industry in view of flexibilization ofthe devices including such layers, and in terms of industrialization andmass productivity mentioned above.

However, for example, the thickness of a layer formed in accordance witha printing method (in particular, by the screen printing method) differsfrom the thickness (typically in submicrons) of a layer (for example, anetching mask) required in manufacture of the aforementioned variousdevices such as semiconductor elements. Specifically, a relatively thicklayer is formed upon patterning by a printing method, but the thicknessrequired for an etching mask may significantly vary depending on thematerial or thickness of a film to be etched.

Meanwhile, a paste or a solution used in a printing method is supposedto have a viscosity suitable for patterning, and thus the viscositythereof is adjusted with a polymer compound. The inventors of thisapplication recognized that when an etching mask used for manufacturingvarious devices is formed with a paste or a solution containingaliphatic polycarbonate, for example, by the screen printing method, thestringiness of the aliphatic polycarbonate may not be appropriatelycontrolled. This, therefore, may result in a problem in that a patternof an etching mask which is good enough for use in the productionprocesses of the aforementioned various devices, or suitable formanufacturing the aforementioned various devices may not be formed. As amore specific example, in a process of forming a layer of a precursor ofan etching mask (hereinafter may also be referred to as an “etching maskprecursor”) by a printing method, a portion of the precursor may befibrously withdrawn from the precursor layer which is formed on a layerto be etched and will form a pattern of the etching mask. This maydisadvantageously disrupt the desired pattern.

Further, when a pattern of the aforementioned etching mask precursor isformed, for example, by a printing method, the pattern which has formedneeds to be maintained at least until a material or region to be etchedis removed. Therefore, formation of a pattern which is resistant totime-dependent change is also one of the challenges for, in particular,the printing method.

Meanwhile, the development of an etching mask for forming a pattern of athin layer used for various devices such as semiconductor elementswithout impairing a function thereof is also another elementaltechnology for achieving the thin layer.

Solutions to the Problems

The inventors of this application have conducted extensive studies andanalyses to develop various metal oxides which can be formed from liquidmaterials as well as an etching mask which can reliably provide apattern of such a metal oxide and can also protect that metal oxide asmuch as possible without impairing a function thereof. Morespecifically, the inventors of this application conducted extensivestudies and analyses to develop an elemental technology and variousdevices manufactured using the elemental technology, which can providean etching mask capable of forming a pattern by a printing method (inparticular, by the screen printing method), and in addition can reliablymaintain or improve the functions of various layers, or can simplify aprocess of producing these layers using the etching mask.

In the studies and analyses conducted by the present inventors, aprocess in which a gel-like layer (hereinafter may also be referred toas a “gel layer”) which can be obtained from the aforementioned paste orsolution formed a pattern was investigated. As a result, the followingfinding was obtained: before aliphatic polycarbonate in a gel layerundergoes decomposition, i.e., before aliphatic polycarbonate is heatedto a temperature at which the aliphatic polycarbonate is decomposed, theproperties of the aliphatic polycarbonate itself or those of the pasteor solution containing the aliphatic polycarbonate have some impacts onthe height, printed conditions, or stringiness of a pattern of a gellayer when the pattern is formed, in particular, by the screen printingmethod. It is noted that in the present application, a gel layersuitable for screen printing is desired, and thus the present gel layermay also be referred to as a gel layer closer to the liquid state.

As a result of numerous trials and errors as well as numerous extensiveanalyses, the inventors of this application found that inclusion ofaliphatic polycarbonate having a molecular weight in a specific range orhaving a specific stringiness can contribute to formation of a patternhaving a desired thickness which can form an etching mask suitable for,in particular, the screen printing method. In addition, the inventors ofthis application found that the aforementioned desired pattern of a gellayer with a controllable thickness can be easily formed by a low-energymanufacturing process such as the screen printing method.

More interestingly, the inventors of this application found that arectification behavior as an electrical property unique tosemiconductors may be lost or impaired when an etching target is anoxide semiconductor, and a conventional resist mask is used as anetching mask for forming a pattern. The inventors of this applicationalso found that in contrast, the rectification behavior of an oxidesemiconductor which had been protected by an etching mask was able to beobserved in a consistent manner when the aforementioned desired patternof a gel layer with a controllable thickness was used as the etchingmask. It is noted that the etching targets in the present inventioninclude not only oxide semiconductors but also oxide conductivematerials or oxide insulators, or other semiconductor materials,conductor materials, and insulator materials.

Therefore, in a more preferred aspect, an oxide (for example, a metaloxide) is used in combination of the aforementioned etching mask, theoxide being formed by annealing an oxide precursor as a startingmaterial with which a pattern is difficult to be formed, for example, bythe screen printing method to or above a temperature at which the oxideis formed, i.e., annealing it to or above a temperature at which anelement dispersed in a solution containing aliphatic polycarbonate bindswith oxygen. This is because a temperature at which the oxide is formedis higher than a temperature at which the etching mask is decomposed,and thus the etching mask has been already decomposed and removedreliably before the oxide is formed. As a result, at least the followingeffects of (1) and (2) can be provided.

-   -   (1) An effect of simplifying a production process where a step        of removing the etching mask needs not be performed separately.    -   (2) An effect of maintaining or improving, in particular, the        electrical properties of an oxide where effects on the physical        properties of the oxide due to, for example, exposure to oxygen        plasma or contact (typically immersion) with a chemical liquid        for removing a resist mask can be reduced or eliminated.

That is, each of the above points of view and schemes can contribute tofurther improvement of the performance and manufacture technology ofvarious devices such as semiconductor elements and electronic devicesmanufactured using an etching mask with a controllable thickness formedby a low-energy manufacturing process (in particular, by the screenprinting method).

The present invention was made based on each of the above points of viewand extensive analysis.

It is noted that the term “process from the liquid state to the gelstate” as used herein typically refers to, for example, a state in whicha solvent is removed to some extent (typically 80% or more by the massratio relative to the entire solvent) by heat treatment, but aliphaticpolycarbonate is not substantially decomposed.

The etching mask for screen printing according to one embodiment of thepresent invention includes aliphatic polycarbonate.

The etching mask precursor for screen printing according to oneembodiment of the present invention includes aliphatic polycarbonate.

With regard to the above etching mask precursor and etching mask,aliphatic polycarbonate can serve as a material of an etching mask whichcan be formed by the screen printing method. This per se is a veryuseful and striking effect. Further, the etching mask can be removedeasily by heating at a decomposition temperature or above of aliphaticpolycarbonate. Therefore, this can significantly contribute tosimplifying processes of manufacturing various devices such assemiconductor elements and electronic devices.

It is noted that one preferred example of the above etching mask forscreen printing is formed from an etching mask precursor includingaliphatic polycarbonate in which the percentage of aliphaticpolycarbonate having a molecular weight of 6000 or more and 400000 orless is 80% by mass or more relative to the entire aliphaticpolycarbonate. Further, another preferred example of the above etchingmask for screen printing is formed from an etching mask precursorincluding aliphatic polycarbonate having a value of L/(D×v×η) of 0.25mm⁻¹Pa⁻¹ or more wherein a cylindrical bar made ofpolytetrafluoroethylene and having a diameter “D” is dipped in acollection of aliphatic polycarbonate having a zero-shear viscosity η asmeasured using a rheometer (Model AR-2000EX, TA Instruments), and thelength “L” of a strand withdrawn from the outermost surface of thecollection of the aliphatic polycarbonate is then measured when thecylindrical bar is pulled upwardly at a velocity “v”.

Further, one preferred example of the above etching mask precursor forscreen printing includes aliphatic polycarbonate in which the percentageof aliphatic polycarbonate having a molecular weight of 6000 or more and400000 or less is 80% by mass or more relative to the entire aliphaticpolycarbonate. Further, another preferred example of the above etchingmask precursor for screen printing includes aliphatic polycarbonatehaving a value of L/(D×v×η) of 0.25 mm⁻¹Pa⁻¹ or more wherein acylindrical bar made of polytetrafluoroethylene and having a diameter“D” is dipped in a collection of aliphatic polycarbonate having azero-shear viscosity η as measured using a rheometer (Model AR-2000EX,TA Instruments), and the length “L” of a strand withdrawn from theoutermost surface of the collection of the aliphatic polycarbonate isthen measured when the cylindrical bar is pulled upwardly at a velocity“v”.

When the aforementioned preferred examples of the etching mask forscreen printing or the aforementioned preferred examples of the etchingmask precursor for screen printing are used, the stringiness ofaliphatic polycarbonate can be controlled more reliably in anappropriate manner. As a result, a good pattern of the etching maskprecursor or the etching mask can be formed. It is noted that thestudies and analyses conducted by the inventors of this applicationindicate that when aliphatic polycarbonate included in an etching maskprecursor and an etching mask satisfies a numerical range based on theaforementioned computational expression, the stringiness of thealiphatic polycarbonate can appropriately be controlled for a layerformed, for example, by the screen printing method. Therefore, a goodpattern of the etching mask precursor or the etching mask can be formedwhen the above numerical range is satisfied.

Further, a method of producing an oxide layer according to oneembodiment of the present invention includes an etching-mask formingstep of forming a pattern of an etching mask on an oxide layer by thescreen printing method, the etching mask including aliphaticpolycarbonate; a contact step of, after the etching-mask forming step,contacting the oxide layer with a solution for dissolving a portion theoxide layer not protected by the etching mask; and a heating step of,after the contact step, heating the oxide layer and the etching mask toor above a temperature at which the etching mask is decomposed.

Further, a method of producing an oxide layer according to anotherembodiment of the present invention includes an etching-mask formingstep of forming a pattern of an etching mask including aliphaticpolycarbonate on an oxide precursor layer to be oxidized into the oxidelayer by the screen printing method; a contact step of, after theetching-mask forming step, contacting the oxide precursor layer with asolution for dissolving a portion of the oxide precursor layer notprotected by the etching mask; and a heating step of, after the contactstep, heating the oxide precursor layer and the etching mask to or abovea temperature at which the oxide layer is formed.

Further, a method of producing an oxide layer according to anotherembodiment of the present invention includes an etching-mask formingstep of forming a pattern of an etching mask including aliphaticpolycarbonate on an oxide layer by the screen printing method; anexposure step of, after the etching-mask forming step, exposing theoxide layer to a plasma for etching a portion of the oxide layer notprotected by the etching mask; and a heating step of, after the exposurestep, heating the oxide layer and the etching mask to or above atemperature at which the etching mask is decomposed.

Further, a method of producing an oxide layer according to anotherembodiment of the present invention includes an etching-mask formingstep of forming a pattern of an etching mask including aliphaticpolycarbonate on an oxide precursor layer to be oxidized into the oxidelayer by the screen printing method; an exposure step of, after theetching-mask forming step, exposing the oxide precursor layer to aplasma for etching a portion of the oxide precursor layer not protectedby the etching mask; and a heating step of, after the exposure step,heating the oxide precursor layer and the etching mask to or above atemperature at which the oxide layer is formed.

In each of the aforementioned methods of producing an oxide layer, whenthe oxide layer in a region to be etched is etched using an etchingmask, the etching mask after formation of a pattern of the oxide layercan be removed by the heating step as a relatively easy treatmentregardless of whether the so-called contact step with a solution(typically an immersion step) or the exposure step with plasma is used.Further, the etching mask used in each of the aforementioned productionmethods can be decomposed and removed reliably by virtue of aliphaticpolycarbonate included therein. In particular, when the target ofetching is an oxide precursor layer to be oxidized into an oxide layer,the etching mask is also decomposed and removed during the heating stepof heating the oxide precursor layer to or above a temperature at whichthe oxide layer is formed. As a result, this can provide a uniqueprocess-simplifying effect in which a step of removing the etching maskneeds not be performed separately.

Further, a method of manufacturing a thin film transistor according toanother embodiment of the present invention includes an oxide-layerforming step of forming an oxide layer so that the oxide layer isbrought into contact with a gate electrode through a gate insulator, theoxide layer being formed via the heating step of any one of theaforementioned methods of producing an oxide layer and being protectedby the etching mask.

According to the method of manufacturing a thin film transistordescribed above, the rectification behavior of the oxide layer can bereliably maintained.

The term “metal oxide” in this application conceptually includes anoxide semiconductor, an oxide conductive material, or an oxideinsulator. In addition, an oxide semiconductor, an oxide conductivematerial, and an oxide insulator are relative concepts in view ofelectroconductivity and thus require no strict distinction. If metaloxides are supposed to be of the same type, they can be recognized by aperson skilled in the art as oxide semiconductors or as oxide conductivematerials or oxide insulators, depending on various device requirements.The term “substrate” in this application is not limited to a base in aplate shape but includes a base and a base material in different modes.Furthermore, the term “application” in each of the embodiments to bedescribed later in this application indicates forming a layer on asubstrate in accordance with a low-energy production process such astypically a printing method. Further, the term “metal” in thisapplication encompasses not only typical elemental metals but alsotransition metals.

Effects of the Invention

The etching mask for screen printing according to one embodiment of thepresent invention and the etching mask precursor for screen printingaccording to one embodiment of the present invention can sufficientlyfunction as an etching mask. In addition, the etching mask can be easilyremoved by heating to a decomposition temperature or above of aliphaticpolycarbonate. Therefore, this can significantly contribute tosimplifying processes of manufacturing various devices such assemiconductor elements and electronic devices.

Further, in the methods of producing an oxide layer according to oneembodiment of the present invention, when a region to be etched of theoxide layer is etched using an etching mask, the etching mask afterformation of a pattern of the oxide layer can be removed by the heatingstep as a relatively easy treatment regardless of whether the so-calledcontact step with a solution (typically an immersion step) or theexposure step with plasma is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows typical examples of optical photomicrographs of goodpatterns realized according to a first embodiment of the presentinvention.

FIG. 2A shows typical examples of optical photomicrographs ofunfavorable patterns formed according to the first embodiment of thepresent invention.

FIG. 2B shows typical examples of optical photomicrographs of goodpatterns formed according to the first embodiment of the presentinvention.

FIG. 3A is a graph showing the relationship between each sample andevaluation parameters indicating stringiness according to the firstembodiment of the present invention.

FIG. 3B is a graph showing the relationship between each sample andevaluation parameters indicating stringiness according to the firstembodiment of the present invention.

FIG. 4A is a graph showing the contact angle between a base material anda solution and the expansion ratio of the solution on the base materialwith respect to changes in the concentration of 2-nitropropane at 30seconds after the placement of the solution containing aliphaticpolycarbonate on the base material according to the first embodiment ofthe present invention.

FIG. 4B is a graph showing the contact angle between a base material anda solution and the expansion ratio of the solution on the base materialwith respect to changes in the concentration of 2-nitropropane at 120seconds after the placement of the solution containing aliphaticpolycarbonate on the base material according to the first embodiment ofthe present invention.

FIG. 5 is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to a second embodiment of thepresent invention.

FIG. 6 is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 7 is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 8 is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 9A is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 9B is a schematic sectional view showing a process in a method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 9C shows an image (a plan view photograph) representing a processdescribed with reference to FIG. 9B according to the present invention.

FIG. 9D is a schematic sectional view showing a process in the method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 9E is an image (a plan view photograph) representing a processdescribed with reference to in FIG. 9D according to the presentinvention.

FIG. 10A is a schematic sectional view showing a process in the methodof producing a thin film transistor according to the second embodimentof the present invention.

FIG. 10B is an image (a plan view photograph) representing a processdescribed with reference to FIG. 10A according to the present invention.

FIG. 11 is a graph showing the TG-DTA properties of an indium andzinc-containing solution as an example of a material of an oxidesemiconductor precursor for forming a channel of the thin filmtransistor according to the second embodiment of the present invention.

FIG. 12 is a graph showing the TG-DTA properties of a solution ofpolypropylene carbonate as an example of a solution containing onlyaliphatic polycarbonate as a solute for forming a component of the thinfilm transistor according to the second embodiment of the presentinvention (a typical example of an etching mask precursor).

FIG. 13 is a graph showing the rectification behavior of a channelconstituting a part of the thin film transistor according to the secondembodiment of the present invention.

FIG. 14 is a schematic sectional view showing a process in the method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 15 is a schematic sectional view showing a process in the method ofproducing a thin film transistor according to the second embodiment ofthe present invention.

FIG. 16A is a schematic sectional view showing a process in the methodof producing a thin film transistor according to the modificationexample (1) of the second embodiment of the present invention.

FIG. 16B is a schematic sectional view showing a process in the methodof producing a thin film transistor according to the modificationexample (1) of the second embodiment of the present invention.

FIG. 17A is a plan view photograph showing the result from etchingtreatment of a chromium (Cr) layer using the etching mask according tothe first embodiment in the alternative embodiments (1) of the presentinvention.

FIG. 17B is a plan view photograph showing the result from etchingtreatment of a chromium (Cr) layer using an etching mask (a known resistmask) as Comparative Example of FIG. 17A.

FIG. 18 is a plan view photograph showing the result from etchingtreatment of a silicon dioxide (SiO₂) layer using the etching maskaccording to the first embodiment in the alternative embodiments (2) ofthe present invention.

DESCRIPTION OF REFERENCE SIGNS

-   10 Substrate-   24 Gate electrode-   32 Gate insulator precursor layer-   34 Gate insulator-   42 Channel precursor layer-   44 Channel-   50 ITO layer-   56 Drain electrode-   58 Source electrode-   80 Etching mask-   100 Thin film transistor

EMBODIMENTS OF THE INVENTION

An etching mask precursor, an etching mask, an oxide precursor, an oxidelayer, a semiconductor element, and an electronic device according toembodiments of the present invention, and a method of producing thesewill be described in detail with reference to the accompanying drawings.It is noted that in this disclosure, common parts are denoted by thesame reference numbers throughout the drawings unless otherwise stated.Further, components in the drawings used in the present embodiments arenot necessarily illustrated in accordance with relative scaling.Moreover, some of the reference numbers may be omitted for clarity ofthe drawings.

First Embodiment

1. Etching Mask Precursor, Configurations of Etching Mask, and Method ofProducing these

The “etching mask precursor” according to this embodiment is in a statewhere aliphatic polycarbonate (which may include inevitable impurities.The same shall apply hereafter) is dissolved in a certain solvent(typically an organic solvent). Further, the “etching mask” according tothis embodiment is in a state (typically a “gel state”) where thesolvent has been removed by heating the etching mask precursor thereofto an extent where it can be used for screen printing.

The etching mask precursor or etching mask according to this embodimentmainly includes aliphatic polycarbonate, but may include a compound,composition, or material other than aliphatic polycarbonate. It is notedthat there is no particular limitation for the lower limit of thecontent of aliphatic polycarbonate in the etching mask precursor oretching mask, but the mass percentage of aliphatic polycarbonate to thetotal amount of a solute is typically 80% or more. Further, there is noparticular limitation for the upper limit of the content of aliphaticpolycarbonate in the etching mask precursor or etching mask, but themass percentage of aliphatic polycarbonate to the total amount of asolute is typically 100% or less. In addition, after the target ofetching is subjected to etching treatment using a pattern formed, forexample, by the screen printing method, aliphatic polycarbonate is to bedecomposed and/or removed primarily in a heating step.

With regard to the etching mask precursor and etching mask according tothis embodiment, aliphatic polycarbonate can serve as a material of anetching mask which can be formed by the screen printing method. This perse is a very useful and striking effect. As described above, the etchingmask can be removed very easily by heating at a decompositiontemperature or above of aliphatic polycarbonate. This can significantlycontribute to simplifying processes for manufacturing various devicessuch as semiconductor elements and electronic devices. Further, in orderto reliably remove the etching mask by heating at a decompositiontemperature or above of aliphatic polycarbonate, no other compounds,compositions, or materials having a decomposition temperature higherthan that of aliphatic polycarbonate are preferably included in theetching mask precursor solution or in the etching mask.

In this embodiment, the percentage of aliphatic polycarbonate having amolecular weight of 6000 or more and 400000 or less in the etching maskprecursor or the etching mask is 80% by mass or more relative to theentire aliphatic polycarbonate.

(Aliphatic Polycarbonate and Solution Containing the AliphaticPolycarbonate)

Next, with regard to aliphatic polycarbonate used in this embodiment,aliphatic polycarbonate and a solution containing the aliphaticpolycarbonate (that is, the “etching mask precursor”) will be describedin detail.

In this embodiment, endothermically decomposable aliphatic polycarbonatehaving good pyrolysability is used. It is noted that whether the thermaldecomposition reaction of aliphatic polycarbonate is an endothermicreaction or not can be determined by the differential thermal analysis(DTA). Such aliphatic polycarbonate has a high oxygen content, and canbe decomposed into low-molecular weight compounds at a relatively lowtemperature, and thus actively contributes to reduction of the amount ofresidual impurities in a metal oxide such as carbonous impurities.

Further, in this embodiment, there is no particular limitation for theorganic solvent which can be used for the “etching mask precursor” as asolution containing aliphatic polycarbonate, as long as it is an organicsolvent in which aliphatic polycarbonate can be dissolved. Specificexamples of the organic solvent include diethylene-glycol-monoethylether acetate (hereinafter, may also be referred to as “DEGMEA”),α-terpineol, β-terpineol, N-methyl-2-pyrrolidone, 2-nitropropane,isopropyl alcohol, diethylene-glycol-monobutyl ether acetate, diethyleneglycol monobutyl ether, toluene, cyclohexane, methyl ethyl ketone,dimethyl carbonate, diethyl carbonate, propylene carbonate, and thelike. Among these organic solvents, preferably used arediethylene-glycol-monoethyl ether acetate, α-terpineol,N-methyl-2-pyrrolidone, 2-nitropropane, and propylene carbonate in viewof their suitably high boiling points and low vaporizability at roomtemperature. It is noted that in this embodiment, the organic solventwill be eventually removed as an unwanted substance after etching thetarget of etching using an etching mask including aliphaticpolycarbonate. Therefore, a solvent mixture of DEGMEA and 2-nitropropaneis preferably used because a pattern is only required to persist for arelatively short time from pattern formulation to decomposition orremoval.

Further, a dispersing agent, a plasticizing agent, and the like canfurther be added, if desired, to the etching mask precursor as asolution containing aliphatic polycarbonate.

Specific examples of the dispersing agent include:

polyhydric alcohol esters such as glycerin and sorbitan;

polyether polyols such as diethylene glycol, triethylene glycol,dipropylene glycol, tripropylene glycol, polyethylene glycol, andpolypropylene glycol; amines such as polyethyleneimine;

(meth)acrylic resins such as polyacrylic acid and polymethacrylic acid;

copolymers of isobutylene or styrene with maleic anhydride, and aminesalts thereof; and the like.

Specific examples of the plasticizing agent include polyether polyol,phthalate ester, and the like.

Further, there is no particular limitation for the method of forming anetching mask precursor layer according to this embodiment. In onepreferred aspect, a layer is formed by a low-energy manufacturingprocess. More specifically, an etching mask precursor layer ispreferably formed by application to a base material in accordance withscreen printing as a particularly simple method.

(Aliphatic Polycarbonate)

In each experimental example as described later, polypropylene carbonateis adopted as an example of aliphatic polycarbonate, but examples of thealiphatic polycarbonate adopted in this embodiment are not particularlylimited. For example, according to a preferably adoptable aspect of thisembodiment, aliphatic polycarbonate obtained by polymerization reactionbetween epoxide and carbon dioxide is used. Adoption of such aliphaticpolycarbonate obtained by polymerization reaction between epoxide andcarbon dioxide exerts an effect such that aliphatic polycarbonate havinga desired molecular weight can be obtained, by which endothermicdecomposition properties can be improved through the control of thestructure of the aliphatic polycarbonate. The aliphatic polycarbonate ispreferably at least one member selected from the group consisting of apolyethylene carbonate and a polypropylene carbonate in terms of a highoxygen content and decomposition into a low molecular weight compound ata relatively low temperature. With the use of any aliphaticpolycarbonate described above having a molecular weight within the abovenumeric range, effects similar to those of this embodiment can beexerted.

The epoxide is not particularly limited if the epoxide undergoes apolymerization reaction with carbon dioxide to form aliphaticpolycarbonate having a structure including aliphatic groups on the mainchain. Adoptable examples of the epoxide in this embodiment includeethylene oxide, propylene oxide, 1-butene oxide, 2-butene oxide,isobutylene oxide, 1-pentene oxide, 2-pentene oxide, 1-hexene oxide,1-octene oxide, 1-decene oxide, cyclopentene oxide, cyclohexene oxide,styrene oxide, vinylcyclohexene oxide, 3-phenylpropylene oxide,3,3,3-trifluoropropylene oxide, 3-naphthylpropylene oxide,3-phenoxypropylene oxide, 3-naphthoxypropylene oxide, butadienemonoxide, 3-vinyloxypropylene oxide, and 3-trimethylsilyloxypropyleneoxide. Among these epoxides, ethylene oxide and propylene oxide arepreferably used in terms of their high polymerization reactivity withcarbon dioxide. These epoxides may each be used singly or may be used incombination of two or more thereof.

The aliphatic polycarbonate mentioned above has a mass-average molecularweight of preferably from 5000 to 1000000 and more preferably from 10000to 500000. The aliphatic polycarbonate having a mass-average molecularweight of less than 5000 may become unsuitable as a material for thescreen printing method because of influences due to a decrease inviscosity, for example. The aliphatic polycarbonate having amass-average molecular weight of more than 1000000 may also becomeunsuitable as a material for the screen printing method due to lowerdissolubility of the aliphatic polycarbonate into an organic solvent. Inaddition, the numerical values of the mass-average molecular weight canbe calculated by the following method.

Specifically, a chloroform solution containing the aliphaticpolycarbonate at a concentration of 0.5% by mass is prepared andmeasured using high performance liquid chromatography. After themeasurement, the molecular weight is calculated by comparison withpolystyrene having a known mass-average molecular weight measured underthe identical conditions. The measurement conditions are as follows.

Type: HLC-8020 (Tosoh Corporation)

Column: GPC column (Trade name: TSK GEL Multipore HXL-M, TosohCorporation)

Column temperature: 40° C.

Eluent: chloroform

Flow speed: 1 mL/minute

The aliphatic polycarbonate mentioned above can be exemplarily producedby polymerization reaction between the epoxide and carbon dioxide in thepresence of a metal catalyst.

A production example of aliphatic polycarbonate is as follows.

The atmosphere of the system of an autoclave having a volume of 1 L andequipped with a stirrer, a gas introduction tube, and a thermometer waspreliminarily replaced with an atmosphere of nitrogen, and was thencharged with a reaction solution containing an organozinc catalyst,hexane, and propylene oxide. The atmosphere of the reaction system wasthen replaced with an atmosphere of carbon dioxide by adding carbondioxide with stirring, and the autoclave was filled with carbon dioxideuntil the pressure of the reaction system reached about 1.5 MPa. Thetemperature of the autoclave was subsequently raised to 60° C. andpolymerization reaction was carried out for several hours whilesupplying carbon dioxide to be consumed by the reaction. Aftercompletion of the reaction, the autoclave was cooled and depressurizedand its content was filtrated. The filtrated product was then driedunder reduced pressure to obtain polypropylene carbonate.

Specific examples of the metal catalyst include an aluminum catalyst anda zinc catalyst. Among these metal catalysts, the zinc catalyst ispreferably used in terms of high polymerization activity inpolymerization reaction between epoxide and carbon dioxide. Anorganozinc catalyst is particularly preferred among the zinc catalysts.

Specific examples of the organozinc catalyst include:

organozinc catalysts such as zinc acetate, diethyl zinc, and dibutylzinc; and

organozinc catalysts obtained by reaction between a zinc compound andcompounds such as primary amine, dihydric phenol, divalent aromaticcarboxylic acid, aromatic hydroxy acid, aliphatic dicarboxylic acid, andaliphatic monocarboxylic acid.

According to a preferred aspect, an organozinc catalyst among theseorganozinc catalysts is adopted, which is obtained by reaction of a zinccompound with an aliphatic dicarboxylic acid and an aliphaticmonocarboxylic acid, due to higher polymerization activity.

A production example of an organozinc catalyst is as follows.

A four-necked flask equipped with a stirrer, a nitrogen gas introductiontube, a thermometer, and a reflux condenser was charged with zinc oxide,glutaric acid, acetic acid, and toluene. After the atmosphere of thereaction system was replaced with an atmosphere of nitrogen, thetemperature of the flask was raised to 55° C. and the materials werestirred at this temperature for 4 hours so as to perform reaction of therespective materials. The temperature of the flask was then raised to110° C. and the materials were stirred at this temperature for 4 hoursso as to cause azeotropic dehydration for removal of only water. Theflask was then cooled to room temperature to obtain a reaction solutioncontaining an organozinc catalyst. The organozinc catalyst, which wasobtained by partially fractionating and filtrating the reactionsolution, was subjected to IR measurement (Thermo Nicolet Japan Inc.,trade name: AVATAR360). As a result, no peak based on carboxylic groupswas confirmed.

The amount of the metal catalyst used for the polymerization reaction ispreferably 0.001 to 20 parts by mass and more preferably 0.01 to 10parts by mass with respect to 100 parts by mass of the epoxide. Thepolymerization reaction may be unlikely to progress if the amount of theused metal catalyst is less than 0.001 parts by mass. In contrast, ifthe amount of the used metal catalyst exceeds 20 parts by mass, effectsin accord with the amount of use may not be obtained, which may beeconomically undesirable.

A reaction solvent to be used as necessary in the polymerizationreaction is not particularly limited. Any type of an organic solvent canbe adopted as the reaction solvent. Specific examples of the organicsolvent include:

aliphatic hydrocarbon solvents such as pentane, hexane, octane, decane,and cyclohexane;

aromatic hydrocarbon solvents such as benzene, toluene, and xylene;

halogenated hydrocarbon solvents such as chloromethane, methylenedichloride, chloroform, carbon tetrachloride, 1,1-dichloroethane,1,2-dichloroethane, ethyl chloride, trichloroethane, 1-chloropropane,2-chloropropane, 1-chlorobutane, 2-chlorobutane,1-chloro-2-methylpropane, chlorobenzene, and bromobenzene; and carbonatesolvents such as dimethyl carbonate, diethyl carbonate, and propylenecarbonate.

The amount of the used reaction solvent is preferably 500 parts by massor more and 10000 parts by mass or less with respect to 100 parts bymass of the epoxide in terms of smooth reaction.

The method of the reaction between an epoxide and carbon dioxide in thepresence of a metal catalyst is not particularly limited in thepolymerization reaction described above. For example, an adoptablemethod includes charging in an autoclave the epoxide, the metalcatalyst, and a reaction solvent as necessary, mixing these components,and then injecting carbon dioxide under pressure for reaction.

The pressure under which the carbon dioxide is used in thepolymerization step is not particularly limited. Typically, the pressureis preferably from 0.1 MPa to 20 MPa, more preferably from 0.1 MPa to 10MPa, and even more preferably from 0.1 MPa to 5 MPa. If the carbondioxide is used at a pressure exceeding 20 MPa, effects in accord withthe amount of use may not be obtained, which may be economicallyundesirable.

The polymerization reaction temperature in the above polymerizationreaction is not particularly limited. The typical polymerizationreaction temperature is preferably from 30° C. to 100° C. and morepreferably from 40° C. to 80° C. If the polymerization reactiontemperature is lower than 30° C., the polymerization reaction may take along period. In contrast, if the polymerization reaction temperatureexceeds 100° C., side reaction may occur with a lower yield. The periodof the polymerization reaction is typically preferred to be from 2 to 40hours, although the period differs depending on the polymerizationreaction temperature and cannot be determined generally.

After the completion of the polymerization reaction, the aliphaticpolycarbonate may be obtained by filtration or the like, and washingwith a solvent or the like as necessary, followed by drying.

[Correlation Between Molecular Weight of Aliphatic Polycarbonate andStringiness and Shape of Pattern]

The inventors of this application extensively studied and analyzed thecorrelation between the molecular weight of aliphatic polycarbonate andthe stringiness and the shape of a pattern typically by conducting thefollowing experiments. As a result, the inventors of this applicationfound that the stringiness of an etching mask in the “gel state” can becontrolled, and a good pattern can be formed when the percentage ofaliphatic polycarbonate having a molecular weight of 6000 or more and400000 or less as a representative example of the molecular weight ofaliphatic polycarbonate is 80% by mass or more relative to the entirealiphatic polycarbonate. It is noted that a type of aliphaticpolycarbonate used in the following Experimental Examples ispolypropylene carbonate (hereinafter may also be referred to as “PPC”).

Experimental Example 1

As examples of aliphatic polycarbonate, samples of the etching maskprecursors in which at least one of 4 types of PPCs having differentmass-average molecular weights was dissolved were prepared as shown inthe following (1) to (10). It is noted that each of the followingmass-average molecular weights represents values for aliphaticpolycarbonate only.

(1) Only a PPC having mass-average molecular weight of 30000(hereinafter may also be referred to as “Sample A”)

(2) Only a PPC having mass-average molecular weight of 90000(hereinafter may also be referred to as “Sample B”)

(3) A 1:1 mixture of Sample A and Sample B (hereinafter may alsoreferred to as “Sample AB”)

(4) Only a PPC having mass-average molecular weight of 230000(hereinafter may also be referred to as “Sample C”)

(5) Only a PPC having mass-average molecular weight of 590000(hereinafter may also be referred to as “Sample D”)

(6) A 1:1 mixture of Sample C and Sample D (hereinafter may alsoreferred to as “Sample CD”)

(7) A 1:1 mixture of Sample A and Sample C (hereinafter may alsoreferred to as “Sample AC”)

(8) A 1:1 mixture of Sample B and Sample C (hereinafter may alsoreferred to as “Sample BC”)

(9) A 1:1 mixture of Sample A and Sample D (hereinafter may alsoreferred to as “Sample AD”)

(10) A 1:1 mixture of Sample B and Sample D (hereinafter may also bereferred to as “Sample BD”)

For the above samples, the percentage of aliphatic polycarbonate havinga molecular weight of 6000 or more and 400000 or less was determined asfollows. That is, a solution of 0.5% by mass of aliphatic polycarbonateprepared in chloroform was subjected to high performance liquidchromatography under the following conditions, and compared with apolystyrene having a known molecular weight to determine the molecularweight distribution.

-   -   Model: HLC-8020 (Tosoh Corporation)    -   Column: GPC column (Trade name: TSK GEL Multipore HXL-M, Tosoh        Corporation)    -   Column temperature: 40° C.    -   Eluent: chloroform    -   Flow rate: 1 mL/minute

The ratio of the area corresponding to the region of the molecularweight of 6000 or more and 400000 or less to the area corresponding tothe entire chromatogram region was calculated from a chromatogramobtained by the above method wherein the horizontal axis represents amolecular weight (log of molecular weight) and the vertical axisrepresents an elution rate (dwt/d (log of molecular weight)). Theresults obtained are shown in Table 1.

TABLE 1 Percentage % by mass of aliphatic polycarbonate having amolecular weight of Sample 6000 to 400000  (1) 81  (2) 93  (3) 87  (4)79  (5) 58  (6) 69  (7) 80  (8) 86  (9) 70 (10) 76

Next, the etching mask precursor was subjected to a heating and stirringstep at about 50° C. for about 72 hours under an atmosphere of air, andthen subjected to a defoaming step for about 30 minutes to form anetching mask precursor in the “gel state” (or a gel state closer to aliquid form) having a viscosity suitable for the screen printing method.Subsequently, rectangular patterns of the above etching mask were formedon a glass substrate “Eagle XG” (200×150×0.7 tmm³) by the screenprinting method.

Further, preliminary annealing of the patterns was then performed at150° C. for 30 minutes in an atmosphere of air, and then the patternswere evaluated for stringiness under an optical microscope and an atomicforce microscope (AFM).

The summary of the above experimental results is shown in Table 2.

TABLE 2 Height of Shape of pattern Sample pattern Stringiness (μm)  (1)Good Good 1.4  (2) Good Good 1.5  (3) Good Good 1.8  (4) Good Bad 2.6 (5) Bad Bad 4  (6) Bad Bad Unmeasurable  (7) Good Good 2.1  (8) GoodGood 2.1  (9) Bad Bad 5.3 (10) Bad Bad 5.3

The “shape of pattern” in Table 2 represents the fidelity of thepatterns formed by the printing method. Accordingly, the expression“bad” with regard to “shape of pattern” means a situation in which apattern of the etching mask is not formed to an extent where it can beused for device production. On the other hand, the expression “good”with regard to “shape of pattern” means a situation in which a patternof the etching mask is reproduced to an extent where it can be used fordevice production. Moreover, the expression “bad” with regard to“stringiness” in Table 2 means a state in which a portion of the etchingmask layer with which a pattern has been formed by the printing methodis drawn as a strand from the etching mask layer, and the desiredpattern is disturbed. Furthermore, the expression “good” with regard to“stringiness” means a state in which almost no or completely nostringiness is observed. In addition, “height of pattern” in Table 2indicates the highest height of a pattern as measured under an atomicforce microscope (AFM). Furthermore, the expression “unmeasurable” withregard to “height of pattern” for Sample (6) means a situation in whicha pattern itself was not essentially formed.

In addition to Table 2 above, the results of the above (1), (7), and (8)are shown in FIG. 1, as examples of typical optical photomicrographsshowing the realization of good patterns. The results of the above (5),(9), and (10) are shown in FIG. 2A as examples of typical opticalphotomicrographs showing unfavorable patterns formed as a result ofuncontrolled stringiness.

As depicted in Table 2, FIG. 1, and FIG. 2A, relativelylow-molecular-weight aliphatic polycarbonate, (1) “Sample A”, (2)“Sample B”, and (3) “Sample AB”, and moderate-molecular-weight aliphaticpolycarbonate, (7) “Sample AC” were found to have good “shape ofpattern” and “stringiness”. In particular, Sample C ((4) in Table) isindicated to have “bad” stringiness in Table 2, however, stringiness wasobserved only partially on the pattern.

The thus obtained results indicate that “shape of patterning” or“stringiness” becomes worse due to an increase in molecular weight.Meanwhile, for example, a pattern formed by a screen printing methodpreferably has a “height” at a level or higher. It was also found thatthe adoption of aliphatic polycarbonate having a significantly lowmolecular weight is not preferred in order to achieve “height ofpattern” at a level or higher while maintaining good “shape of pattern”and “stringiness”.

In addition, the inventors of this application consider the cause of“bad” “shape of patterning” and “bad” “stringiness” of (9) “Sample AD”and (10) “Sample BD” as follows.

As depicted in “height of pattern” in Table 2, each height of pattern(5.3 μm) of (9) “Sample AD” and (10) “Sample BD” is almost the same asthe sum of the height of pattern (1.4 μm) of “Sample A” and the heightof pattern (4 μm) of “Sample D”, or, the sum of the height of pattern(1.5 μm) of “Sample A” and the height of pattern (4 μm) of “Sample D”.Accordingly, it is considered that the use of a sample prepared bydissolving only one type of PPC as a binder results in a situation wherehigh-molecular-weight aliphatic polycarbonate and low-molecular-weightaliphatic polycarbonate become immiscible due to namely, phaseseparation, when a difference in molecular weight between the twoexceeds a level. More specifically, there can be a situation wherehigh-molecular-weight aliphatic polycarbonate is placed onlow-molecular-weight aliphatic polycarbonate, or a situation opposite tothat. Meanwhile, (7) “Sample AC” or (8) “Sample BC” having relatively asmall difference in molecular weight were considered to be in a state inwhich the high-molecular-weight aliphatic polycarbonate and thelow-molecular-weight aliphatic polycarbonate are mixed appropriatelywithout undergoing namely, phase separation. This is considered to bethe reason of their good “shape of patterning” and “stringiness”.

Therefore, the above experimental results can be said to indicate thateven when materials of the same kind were used, specifically, whenmultiple types of aliphatic polycarbonate having different mass-averagemolecular weights were used, simple equalization of the mass-averagemolecular weights of the multiple types of aliphatic polycarbonatecannot always lead to a molecular weight appropriate for obtaining good“shape of patterning” and “stringiness”.

Each of the above results and the results of examination and analysesmade by the inventors of this application concerning other molecularweights together indicate that good “shape of pattern” and “stringiness”can be realized by adopting aliphatic polycarbonate having a molecularweight of 6000 or more and 400000 or less and constituting 80% by massor more of the entire aliphatic polycarbonate.

Furthermore, the results from (1) to (10) revealed that the data fromthe samples of the oxide semiconductor precursors in which one of the 4types of PPCs having different mass-average molecular weights wasdissolved as a binder, or the samples of the oxide semiconductorprecursors in which two of the PPCs as binders were dissolved incombination showed a similar trend. It is noted that each of the samplesof the oxide semiconductor precursors contains 5% by mass of a solutioncontaining 0.2 mol/kg of indium. Moreover, the sample numbers used inTable 3 are the same as those used in Table 1 for clarifying thecorrespondence relationship with (1) to (10) in Table 1.

Production examples of oxide precursors are as follows. It is noted thatin each of the following experimental examples, an oxide semiconductorprecursor, i.e., a precursor to be oxidized into an oxide semiconductoris used as a representative example.

First, indium acetylacetonate and propionic acid were gradually mixed ina 50 mL flask with stirring to obtain an indium-containing solutionwhich will eventually form indium oxide.

Next, polypropylene carbonate was dissolved in a solvent mixture ofDEGMEA and 2-nitropropane in a 50 mL eggplant-shaped flask to obtain asolution of 25 wt % polypropylene carbonate.

Subsequently, the above indium-containing solution was gradually addedto the above solution of polypropylene carbonate to obtain the aboveoxide semiconductor precursor.

Experimental results shown in Table 3 will be described. (1) “Sample A”,(2) “Sample B”, and (3) “Sample AB” as aliphatic polycarbonate having arelatively low molecular weight, and (7) “Sample AC” as aliphaticpolycarbonate having a medium molecular weight were found to show good“shape of pattern” and “stringiness”. Interestingly, Samples (9) and(10) having a relatively large molecular weight were also found to showgood pattern shapes and stringiness. FIG. 2B represents an image takenunder an optical microscope showing the results from (9) and (10) inwhich good patterns were formed as a result of successful control ofstringiness. The good pattern shapes and stringiness for the samples (9)and (10) can likely be explained in part by a somewhat decreased overallconcentration of PPC due to addition of the indium-containing solutionto the samples used in Table 2. However, this does not appear to be thepredominant factor.

TABLE 3 Height of Shape of pattern Sample pattern Stringiness (μm)  (1)Bad Good 1  (2) Bad Good Unmeasurable  (3) Good Good 1.2  (4) Bad Bad4.4  (5) Bad Bad Unmeasurable  (6) Bad Bad Unmeasurable  (7) Good Good3.2  (8) Good Good 2.6  (9) Good Good 3.7 (10) Good Good 5.3

Experimental Example 2

Next, in the following experiment, the inventors of this applicationmeasured the average length of drawn out strands (mm) and zero-shearviscosity (Pa·s) of the samples (1) to (10) used in experimental example1.

In this experiment, first, regarding the average length of drawn-outstrands (mm), a cylindrical bar made of polytetrafluoroethylene andhaving a diameter “D” of 2.9 mm was dipped in a collection of aliphaticpolycarbonate, which had been formed using each sample, within acontainer. When the cylindrical bar was then raised at a velocity “v” of5 mm/second, and then the length “L” (mm) of a strand drawn out from theoutermost surface of the collection of the aliphatic polycarbonate wasmeasured. In addition, the zero-shear viscosity η of each sample wasmeasured using a rheometer (TA Instruments, AR-2000EX). Each of theabove values was substituted as an evaluation parameter into thefollowing formula for calculation.

“Evaluation parameter” (mm⁻¹·Pa⁻¹)=L/(D×v×η)  <Formula>

Table 4 shows the relationship between each sample, and average lengthof drawn-out strands (mm) and zero-shear viscosity (Pa·s) in thisExperimental example. FIG. 3A is a graph showing the relationshipbetween each sample and evaluation parameter representing stringiness inthis Experimental example.

TABLE 4 Average length of Zero-shear Evaluation drawn-out strandsviscosity parameter Sample (mm) (Pa · s) (mm⁻¹ · Pa⁻¹)  (1) 2.62 0.141.31  (2) 2.74 0.41 0.47  (3) 2.59 0.27 0.67  (4) 113.51 58.7 0.13  (5)129.58 392.5 0.02  (6) 124.77 279.4 0.03  (7) 51.37 5.7 0.62  (8) 54.807.7 0.49  (9) 38.36 11.8 0.22 (10) 65.45 29.4 0.15

The results in Table 4 and FIG. 3A showed that for a sample prepared bydissolving only one type of PPC as a binder, good “shape of patterning”and “stringiness” can be obtained when the value of the “evaluationparameter” (mm⁻¹·Pa⁻¹) calculated by the above formula is 0.4 mm⁻¹Pa⁻¹or more.

Meanwhile, as indicated in Table 5 below, the results from (1) to (10)showed that the data from the samples of the oxide semiconductorprecursors in which only one of the 4 types of PPCs having differentmass-average molecular weights was dissolved as a binder, or the samplesof the oxide semiconductor precursors in which two of the PPCs asbinders were dissolved in combination had a similar trend. It is notedthat the sample numbers used in Table 5 are the same as those used inTable 1 for clarifying the correspondence relationship with (1) to (10)in Table 1.

As shown in Tables 4 and 5 and FIGS. 3A and 3B, the “evaluationparameter” generally showed a larger value when the above samples of theoxide semiconductor precursors were used as compared with a case wheresamples having only one type of PPC dissolved as a binder were used.However, the inventors of this application assume that an impact ofindium acetylacetonate, which is a solute contained in a very smallamount, is very small. That is, the inventors of this applicationconclude that the results shown in Table 4 and FIG. 3A representauthentic and more generalizable experimental results.

Therefore, the results in Tables 3 to 5 and FIGS. 3B and 2B takentogether for analysis based on the results from (9) and (10) in Table 5revealed that good “shape of patterning” and “stringiness” were able tobe obtained when the value of the “evaluation parameter” (mm⁻¹·Pa⁻¹) was0.25 mm⁻¹·Pa⁻¹ or more (more strictly, 0.29 mm⁻¹·Pa⁻¹ or more). It isnoted that there is no particular limitation for the upper limit of the“evaluation parameter” for obtaining good “shape of patterning” and“stringiness,” but it is preferably 1.2 or less (more strictly, 0.9 orless) in order to obtain the height of a pattern more reliably.

TABLE 5 Average length of Zero-shear Evaluation drawn-out strandsviscosity parameter Sample (mm) (Pa · s) (mm⁻¹ · Pa⁻¹)  (1) 2.33 0.101.56  (2) 2.65 0.29 0.64  (3) 2.57 0.21 0.84  (4) 92.71 54.9 0.12  (5)126.65 340.5 0.03  (6) 115.49 96.9 0.08  (7) 29.69 2.5 0.83  (8) 36.473.4 0.73  (9) 30.26 7.2 0.29 (10) 65.96 15.6 0.29

(Other Preferable Molecular Weight Ranges of Aliphatic Polycarbonate)

In addition, the molecular weight range of aliphatic polycarbonate inthis embodiment is not limited to the numeric range disclosed in each ofthe above experimental examples. As a result of analyses made by theinventors of this application, for example, aliphatic polycarbonatehaving a molecular weight of 6000 or more and 300000 or less constitutes75% by mass or more of the entire aliphatic polycarbonate in order tomore reliably control stringiness and form a good pattern in a furtherpreferred aspect.

Experimental Example 3 (Evaluation of Contact Angle and Expansion Ratio)

As already described, aliphatic polycarbonate which can serve as anetching mask is to be decomposed and/or removed primarily in a heatingstep after the target of etching is etched using a pattern formed, forexample, by the screen printing method. Therefore, the etching mask isonly required for a limited time period. Nonetheless, the inventors ofthis application assumed that evaluation of the contact angle between anetching mask precursor and a base material when a solution containingaliphatic polycarbonate (that is, an “etching mask precursor”) is placedon the base material can provide an indicator suitable for determiningwhether or not the solution has a pattern formability (in other words,well-balanced viscosity or wettability) enough for persisting during thelimited time period.

Accordingly, the inventors of this application conducted the followingstudy: a solution of polypropylene carbonate obtained by dissolvingpolypropylene carbonate (25 wt %) in a solvent mixture of DEGMEA and2-nitropropane was placed on a base material (in this experimentalexample, a glass substrate), and then the contact angle between the basematerial and the solution and the expansion ratio of the solution on thebase material were determined at 30 seconds and 120 seconds afterplacement to investigate how they changed over time. It is noted thatthe above evaluation was performed as a function of the concentration(wt %) of 2-nitropropane, which is a solvent suitable for aliphaticpolycarbonate, in order to facilitate the study about how the contactangle changes. Further, the above “expansion ratio” was calculated basedon the actual size of a pattern with respect to the designed value.

FIG. 4A is a graph showing the contact angle between a base material anda solution, and expansion ratio of the solution on the base materialwith respect to changes in the concentration of 2-nitropropane at 30seconds after placement of the solution containing aliphaticpolycarbonate on the base material. Furthermore, FIG. 4B is a graphshowing the contact angle between a base material and a solution andexpansion ratio of the solution on the base material with respect tochanges in the concentration of 2-nitropropane at 120 seconds afterplacement of the solution containing aliphatic polycarbonate on the basematerial.

As depicted in FIG. 4A and FIG. 4B, the contact angle between the basematerial and the solution, and expansion ratio of the solution on thebase material were found to be inversely correlated to each other.Specifically, it was revealed that as the concentration of2-nitropropane increased, the contact angle increased and the expansionratio decreased. However, as depicted in FIG. 4B, even when theconcentration of 2-nitropropane was high (for example, 75%), there was acase in which the contact angle did not increase. Moreover, it wasrevealed that particularly the contact angle was observed to tend tosharply increase when the concentration of 2-nitropropane reached alevel (typically, 55% or more and 60% or less), as depicted in both FIG.4A and FIG. 4B.

According to further research and analyses made by the inventors of thisapplication, it was found that a case in which the contact angle exceeds36° at the stage of 30 seconds after placement, or, a case in which thecontact angle exceeds 32° at the stage of 120 seconds after placementresults in a situation where a solution containing aliphaticpolycarbonate is repelled from the base material, making it difficult tosufficiently form a pattern that can be kept for a temporary time.Accordingly, a solution containing aliphatic polycarbonate is preferablyprepared to achieve the above contact angle or less.

On the other hand, the results also revealed that when the contact angleat 30 seconds after the placement was less than 30°, or the contactangle at 120 seconds after the placement was less than 26°, thereproducibility of patterns was poor, and thus a pattern substantiallypersisting for a temporary time was difficult to be formed. Therefore,the solution containing aliphatic polycarbonate is preferably preparedso as to show an angle larger than each of the above contact angles.Considering all of the above, the contact angle of an etching maskprecursor and a base material is preferably 30° or more and 36° or lessat 30 seconds after placement of the etching mask precursor on the basematerial, or the contact angle of an etching mask precursor and a basematerial is preferably 26° or more and 32° or less at 120 seconds afterplacement of the etching mask precursor on the base material.

Furthermore, the results depicted in FIG. 4A and FIG. 4B above wereconfirmed to correlate with the results of samples of the oxidesemiconductor precursor explained in “preparation step for eachexperiment”, which were each prepared by dissolving one of 4 types ofPPC differing in mass-average molecular weight as a binder, or, samplesof the oxide semiconductor precursor prepared by dissolving acombination of any 2 out of the 4 types of the same as a binder.

As described above, it can be confirmed that the result of a solutioncontaining aliphatic polycarbonate, but containing no oxide precursor inthe experimental example was approximately equivalent to that of asolution containing aliphatic polycarbonate and an oxide precursor inthe Example.

2. Oxide Precursor, Configuration of Oxide Layer, and Method ofProducing these

In this embodiment, an “oxide precursor” represents a typical aspect inwhich aliphatic polycarbonate is mixed with a compound of a metal to beoxidized into a metal oxide. Therefore, a typical example of the aboveoxide precursor is a solution containing aliphatic polycarbonate inwhich a compound of a metal to be oxidized into a metal oxide isdispersed. Further, as already described, examples of the above metaloxide include oxide semiconductors, oxide conductive materials, or oxideinsulators. It is noted that once a pattern is formed, for example, bythe screen printing method, aliphatic polycarbonate may be considered asan impurity with respect to the metal oxide finally obtained, and thusit will be decomposed and/or removed primarily in a heating step.

It is noted that examples of the metal oxide in this embodiment includeoxide semiconductors, oxide conductive materials, or oxide insulators.Further, in this embodiment, the percentage of aliphatic polycarbonatehaving a molecular weight of 6000 or more and 400000 or less is 80% bymass or more relative to the entire aliphatic polycarbonate as in thecase of the etching mask precursor or etching mask. In addition,materials and production methods which can be used for the etching maskprecursor or etching mask may also be used except that theaforementioned metal compound is mixed. Moreover, as a typical example,a method of producing an oxide semiconductor precursor and an oxidesemiconductor layer are disclosed in detail in Patent Document 4(WO2015-019771) filed by the applicant of this application. Therefore,descriptions common with the etching mask precursor or etching mask willbe omitted.

Second Embodiment 1. Overall Structure of Thin Film Transistor Accordingto Present Embodiment

FIGS. 5 to 9B, 9D, 10A, 14, and 15 are each a schematic sectional viewof a process in the method of producing a thin film transistor 100 as anexemplary semiconductor element. It is noted that FIG. 15 is a schematicsectional view of a process in the method of producing the thin filmtransistor 100 according to the present embodiment and the overallstructure of the thin film transistor. As shown in FIG. 15, the thinfilm transistor 100 according to the present embodiment includes asubstrate 10, on which a gate electrode 24, a gate insulator 34, achannel 44, a source electrode 58, and a drain electrode 56 are stackedfrom the lower side in the mentioned order. Further, production orimplementation of an electronic device including the above semiconductorelement (e.g. a mobile terminal, an information appliance, or any otherpublicly known electric appliance) can be fully understood without anyfurther explanation by a person skilled in the art who understands thesemiconductor element according to the present embodiment. Further,various steps for forming an oxide precursor layer, which are describedbelow, are included in the “oxide precursor layer forming step” in thisapplication.

The thin film transistor 100 has the so-called bottom gate structure,although this embodiment is not limited to this structure. A personskilled in the art having ordinary technical knowledge can thus form thetop gate structure by changing the order of the steps with reference tothe description of this embodiment. Temperatures indicated in thisapplication are surface temperatures of the heating surface of a heater,which comes into contact with a substrate. Patterning of an extractionelectrode from each electrode is not depicted in order forsimplification of the drawings.

There is no particular limitation for the substrate 10 according to thepresent embodiment, but a substrate commonly used for semiconductorelements may be used. Various base materials may be used, includinghighly heat resistant glass, an SiO₂/Si substrate (i.e. a substrate inwhich an oxide silicon film is formed on a silicon substrate), analumina (Al₂O₃) substrate, an STO (SrTiO) substrate, an insulatingsubstrate in which an STO (SrTiO) layer is formed over a surface of anSi substrate through a SiO₂ layer and a Ti layer, and the like, andsemiconductor substrates (e.g. a Si substrate, a SiC substrate, and a Gesubstrate). It is noted that examples of the insulating substrateinclude films or sheets made of materials such as polyesters includingpolyethylene terephthalate and polyethylene naphthalate; polyolefins;cellulose triacetate; polycarbonate; polyamide; polyimide; polyamideimide; polysulfone; aramid; and aromatic polyamide. Further, there is noparticular limitation for the thickness of a substrate, but it is, forexample, 3 μm or more and 300 μm or less. Moreover, the substrate may behard, or may be flexible.

2. Method of Manufacturing Thin Film Transistor According to PresentEmbodiment (1) Formation of Gate Electrode

In this embodiment, a compound of a metal to be oxidized into an oxideconductive material (hereinafter, may also simply be referred to as an“oxide conductive material”) may be used as a material for the gateelectrode 24. In this case, the gate electrode 24 in this embodiment isformed by annealing a layer of an oxide conductive material precursor(hereinafter, may also be referred to as an “oxide conductive materialprecursor layer”) in which an oxide conductive material (may includeinevitable impurities. The same shall apply to oxides of other materialsin addition to the oxide of this material) is dispersed in a solutioncontaining aliphatic polycarbonate. In this embodiment, as shown in FIG.5, a gate electrode precursor layer 22 can be formed by applying a gateelectrode precursor solution as a starting material onto a SiO₂/Sisubstrate 10 as a base material (hereinafter, may also simply bereferred to as a “substrate”) by a low-energy production process (forexample, a printing method or a spin coating method).

Subsequently, an annealing step for annealing the gate electrodeprecursor layer 22 in air for a predetermined time period (for example,10 minutes to 1 hour) at 450° C. to 550° C. is performed, for example.As a result, as depicted in FIG. 6, the gate electrode 24 is formed onthe substrate 10. In addition, the thickness of the layer of the gateelectrode 24 in this embodiment is about 100 nm, for example.

Here, one example of the above oxide conductive material is a materialhaving a structure (typically a complex structure) in which a ligand iscoordinated with a metal to be oxidized into an oxide conductivematerial. Examples of the oxide conductive material according to thepresent embodiment can include metal organic acid salts, metal inorganicacid salts, metal halides, and various metal alkoxides. It is noted thatan example of a metal to be oxidized into an oxide conductive materialis ruthenium (Ru). In this embodiment, an annealing step is performed toform ruthenium oxide as an oxide conductive material, with which thegate electrode 24 can be formed, the annealing step including annealinga gate electrode precursor solution, for example, in air for apredetermined time period (for example, 10 minutes to 1 hour) at about450° C. to about 550° C., the gate electrode precursor solution beingprepared using a solution as a starting material in which ruthenium(III) nitrosylacetate is dissolved in a solvent mixture of propionicacid and 2-aminoethanol containing aliphatic polycarbonate.

In this embodiment, in particular, when a gate electrode precursorsolution containing the aliphatic polycarbonate described in the firstembodiment is used, a good pattern of the gate electrode precursor layer22 can be formed by a printing method. More specifically, thestringiness of aliphatic polycarbonate in the gate electrode precursorsolution can be controlled in an appropriate manner, leading toformation of a good pattern of the gate electrode precursor layer 22.

In this embodiment, instead of the above gate electrode 24, high meltingmetal such as platinum, gold, silver, copper, aluminum, molybdenum,palladium, ruthenium, iridium, or tungsten, a metal material such as analloy thereof, a pt silicon layer, or an n⁺-silicon layer can beapplied, for example. In this case, the gate electrode 24 can be formedon the substrate 10 in accordance with a known sputtering technique or aknown CVD technique.

(2) Formation of Gate Insulator

Furthermore, in this embodiment, a gate insulator 34 can be formed byannealing a layer of an oxide insulator precursor (hereinafter, may alsobe referred to as an “oxide insulator precursor layer”) in which acompound of a metal to be oxidized into an oxide insulator (hereinaftermay also simply be referred to as an “oxide insulator”) is dispersed ina solution containing aliphatic polycarbonate.

Specifically, as depicted in FIG. 7, a gate insulator precursor layer 32is formed by applying the above oxide insulator precursor onto the gateelectrode 24 in accordance with a low-energy production process (e.g. aprinting method or a spin coating method).

The gate insulator precursor layer 32 in a gel state is then subjectedto the annealing (main annealing) step for annealing at about 450° C. toabout 550° C. for a predetermined time period (e.g. 10 minutes to 1hour) in air, for example, thereby forming an oxide of lanthanum (La)and zirconium (Zr), which is an oxide insulator. As a result, asdepicted in FIG. 8, the gate insulator 34 can be formed. In addition,the thickness of the layer of the gate insulator 34 of this embodimentranges from about 100 nm to about 250 nm, for example.

An example of the above oxide insulator is a material having a structure(typically a complex structure) wherein a ligand is coordinated with ametal to be oxidized into an oxide insulator. Examples of the oxideinsulator of this embodiment can include metal organic acid salts, metalinorganic acid salts, metal halides, and various metal alkoxides, or,other organic acid salts, inorganic acid salts, halides, or variousalkoxides.

It is noted that a typical example of the oxide insulator is an oxideincluding lanthanum (La) and zirconium (Zr). This oxide can be used asthe gate insulator 34. In this embodiment, a first solution is preparedin which lanthanum (III) acetate is dissolved in propionic acid (asolvent) containing aliphatic polycarbonate, and a second solution isprepared in which zirconium butoxide is dissolved in propionic acid (asolvent) containing aliphatic polycarbonate. A gate insulator precursorsolution as a starting material in which the first solution and thesecond solution are mixed can be subjected to an annealing step forannealing, for example, in air for a predetermined time period (forexample, 10 minutes to 1 hour) at about 450° C. to about 550° C. to forman oxide insulator.

In this embodiment, in particular, when an oxide insulator precursorcontaining the aliphatic polycarbonate described in the first embodimentis used, a good pattern of the gate insulator precursor layer 32 can beformed by a printing method. More specifically, the stringiness ofaliphatic polycarbonate in the oxide insulator precursor can becontrolled in an appropriate manner, leading to formation of a goodpattern of the gate insulator precursor layer 32.

Moreover, in this embodiment, oxide silicon or silicon oxynitride can beapplied, for example, instead of the above gate insulator 34. In thiscase, the gate insulator 34 can be formed on the gate electrode 24 by aknown CVD technique, for example.

(3) Formation of Channel

Further, in this embodiment, a channel 44 can be formed by annealing alayer of an oxide semiconductor precursor (hereinafter, may also bereferred to as an “oxide semiconductor precursor layer”) in which acompound of a metal to be oxidized into an oxide semiconductor(hereinafter, may also simply be referred to as “oxide semiconductor”)is dispersed in a solution containing aliphatic polycarbonate. In thisembodiment, a channel precursor layer 42 can be formed by applying achannel precursor solution as a starting material onto the gateinsulator 34 by a low-energy production process (for example, a printingmethod or a spin coating method) as shown in FIG. 9A.

Subsequently, the channel precursor layer 42 is subjected to anannealing step described below to form the channel 44 as shown in FIG.10A.

Here, an example of the oxide semiconductor is a material having astructure (typically a complex structure) in which a ligand iscoordinated with a metal to be oxidized into an oxide semiconductor.Examples of a material to be used for forming the oxide semiconductoraccording to the present embodiment can include metal organic acidsalts, metal inorganic acid salts, metal halides, and various metalalkoxides. It is noted that a typical example of the oxide semiconductoris indium-zinc oxide. For example, indium-zinc oxide (hereinafter, mayalso be referred to as “InZnO”) as an oxide semiconductor can be formedby performing an annealing step of annealing a solution in which indiumacetylacetonate and zinc chloride are dissolved in propionic acidincluding aliphatic polycarbonate (may also be referred to as an “InZnsolution”) at 450° C. to 550° C. for a predetermined time (for example,10 minutes to 1 hour) in air. As a result, the channel 44 can be formed.

Examples of the metal to be oxidized into an oxide semiconductor includeone, two, or more members selected from the group consisting of indium,tin, zinc, cadmium, titanium, silver, copper, tungsten, nickel,indium-zinc, indium-tin, indium-gallium-zinc, antimony-tin, andgallium-zinc. In terms of element performance, stability, and the like,indium-zinc is preferably adopted as the metal to be oxidized into anoxide semiconductor.

In this embodiment, in particular, when a channel precursor solutioncontaining the aliphatic polycarbonate described in the first embodimentis used, a good pattern of the channel precursor layer 42 can be formedby a printing method. More specifically, the stringiness of aliphaticpolycarbonate in the channel precursor solution can be controlled in anappropriate manner, leading to formation of a good pattern of thechannel precursor layer 42.

Moreover, in this embodiment, in particular, when the channel 44 as anoxide semiconductor layer is formed, a method of producing a metal oxideaccording to one embodiment of the invention which has been invented bythe inventors of this application, for example, a method disclosed inPatent Document 4, can be used as a preferred example.

A typical method of forming the channel 44 involves: a precursor layerforming step of forming an oxide semiconductor precursor on or above asubstrate to form a layer, the oxide semiconductor precursor having acompound of a metal to be oxidized into the oxide semiconductordispersed in a solution containing aliphatic polycarbonate; and anannealing step of heating the precursor layer to a first temperature atwhich 90 wt % or more of aliphatic polycarbonate is decomposed, and thenannealing the precursor layer at a temperature higher than the firsttemperature and equal to or higher than a second temperature at whichthe metal binds to oxygen, and where an exothermic peak is observed asdetermined by differential thermal analysis (DTA) of the precursor orthe metal compound.

<TG-DTA (Thermogravimetry and Differential Thermal Analysis) Properties>

More specifically, FIG. 11 is a graph showing the TG-DTA properties ofan indium-zinc-containing solution as an example of a material forforming the oxide precursor according to the first embodiment, whichwill serve as an oxide semiconductor precursor for forming a channel ofa thin film transistor. Further, FIG. 12 is a graph showing an exampleof the TG-DTA properties of a solution of polypropylene carbonate as anexample of a solution (a typical example of an etching mask precursor)containing aliphatic polycarbonate alone as a solute for forming acomponent (for example, a channel) of a thin film transistor. It isnoted that as shown in FIGS. 11 and 12, solid lines in the figuresrepresent results from thermogravimetric (TG) measurements, and dottedlines in the figures represent results from differential thermalanalysis (DTA) measurements.

Results from the thermogravimetric measurements in FIG. 11 showed asignificant weight loss at around 120° C. probably due to evaporation ofthe solvent. The results also showed an exothermic peak at around 330°C. in the graph of the differential thermal analysis of the InZnsolution as indicated by (X) in FIG. 11. This thus indicates that indiumand zinc are bound with oxygen at around 330° C. Therefore, thistemperature of 330° C. corresponds to the aforementioned secondtemperature.

In contrast, the results in FIG. 12 from the thermogravimetricmeasurements showed a significant weight loss at around 140° C. to 190°C. due to partial decomposition or elimination of polypropylenecarbonate itself as the solvent in the solution of polypropylenecarbonate disappeared. It is noted that polypropylene carbonate appearsto be converted into carbon dioxide and water by the abovedecomposition. The results in FIG. 12 also showed that 90 wt % or moreof aliphatic polycarbonate was decomposed and removed at around 190° C.Therefore, this temperature of 190° C. corresponds to the aforementionedfirst temperature. More detailed analysis revealed that 95 wt % or moreof aliphatic polycarbonate was decomposed at around 250° C., and almostall (99 wt % or more) of aliphatic polycarbonate was decomposed ataround 260° C. Therefore, use of an etching mask precursor including atype of aliphatic polycarbonate which can be substantially orpredominantly eliminated or removed by heat treatment at 250° C. or more(more preferably 260° C. or more) can reliably reduce or prevent effectson a layer to be etched (typically, adverse effects on electricalproperties). It is noted that the above results were obtained whenaliphatic polycarbonate was decomposed by relatively brief heattreatment. However, aliphatic polycarbonate was found to besubstantially decomposed even at a lower temperature (for example, 180°C.) when a longer heat treatment is performed. Therefore, use of anetching mask precursor including a type of aliphatic polycarbonate whichcan be substantially or predominantly eliminated or removed by heattreatment at 180° C. or more can reduce or prevent effects on a layer tobe etched (typically, adverse effects on electrical properties).

Further, studies conducted by the inventors of this application showedthat when heated (for example, using a known heater) while applyingultraviolet rays (for example, a low-pressure mercury lamp (Model UV-300H-E, SAMCO) is used, and the ultraviolet rays have wavelengths of 185 nmand 254 nm), the first temperature was able to be lowered by tens ofdegrees (for example, about 30° C.) as compared with a temperature whenheated without use of ultraviolet irradiation. Therefore, in a heatingstep for decomposing aliphatic polycarbonate, heating is preferablyperformed while applying ultraviolet rays to each oxide layer precursorand/or etching mask at or above a temperature where organic substances,for example, the aforementioned organic solvent, the organic portion ofthe aforementioned complex structure, or the aforementioned aliphaticpolycarbonate are decomposed. This is because the final oxide layerpreferably includes the organic portions of the aforementioned solute,the aforementioned solvent, or the aforementioned aliphaticpolycarbonate as less as possible. It is noted that the ultraviolet raysdo not necessarily need to be applied after decomposition of the organicsubstances in the above heating step, but in a more preferred aspect,heating is performed while applying ultraviolet rays to each oxideprecursor layer and/or etching mask at or above a temperature where theoxide layer is formed. It is noted that there is no particularlimitation for the wavelengths of the ultraviolet rays. Ultraviolet rayshaving wavelengths other than 185 nm or 254 nm can show a similareffect.

The temperature (the second temperature) where an exothermic peak isobserved in differential thermal analysis (DTA) at which a metal in anoxide semiconductor binds to oxygen is preferably sufficiently higherthan the temperature at which aliphatic polycarbonate is decomposed, orthe temperature at which aliphatic polycarbonate is decomposed issufficiently lower than the temperature (the second temperature) atwhich the exothermic peak is observed. If that is the case, 90 wt % ormore (more preferably 95 wt % or more, even more preferably 99 wt % ormore, and most preferably 99.9 wt % or more) of aliphatic polycarbonatecan be decomposed more reliably.

Further, an etching mask can be reliably eliminated or removed when heattreatment is performed at a temperature where aliphatic polycarbonate isdecomposed (typically, at the first temperature or above, preferably180° C. or above, more preferably 250° C. or above, and even morepreferably 260° C. or above). Strikingly, this can reliably reduce oreliminate effects on the electrical properties of an oxide (includingnot only a semiconductor but also a conductive material or an insulator)formed by heating at the second temperature or above.

According to the research and analyses conducted by the inventors ofthis application, a difference between the first temperature and thesecond temperature is 10° C. or higher, more preferably 50° C. orhigher, and further preferably 100° C. or higher, leading to thereduction of impurities represented by carbonous impurities remaining inthe oxide layer.

The oxide semiconductor is not particularly limited in terms of itsphase state. For example, the oxide semiconductor may be in any one ofthe crystal form, the polycrystalline form, and the amorphous form. Thepresent embodiment can also cover a phase state where grown crystal hasa branch form or a scale form. Furthermore, the present embodiment isobviously not particularly limited by a patterned shape (e.g. aspherical shape, an elliptical shape, or a rectangular shape).

(Step of Forming and Annealing Pattern of Channel Precursor Layer)

Next, a specific method of forming a pattern of the channel 44 will bedescribed. It is noted that this method of forming a pattern of thechannel 44 can also be applied partially or almost completely tomanufacture of the aforementioned oxide conductive materials or oxideinsulators (that is, various oxide layers).

As already described, in this embodiment, the channel precursor layer 42is formed by applying a channel precursor solution onto the gateinsulator 34 in accordance with a low-energy production process (e.g., aspin coating method) as shown in FIG. 9A. It is noted that there is noparticular limitation for the thickness (wet) of the channel precursorlayer 42 as an oxide semiconductor precursor layer.

Subsequently, a channel precursor layer 42 having a thickness of about600 nm is formed by heating, for example, at 150° C. for a predeterminedperiod (e.g. 3 minutes) in the preliminary annealing (may also bereferred to as the “first preliminary annealing”) step. The firstpreliminary annealing step is performed mainly for the purpose ofpromoting gelation and fixation of the channel precursor layer 42 on thegate insulator 34.

In this embodiment, a pattern of an etching mask 80 formed from theetching mask precursor according to the first embodiment is then formedon the channel precursor layer 42 by the screen printing method (anetching-mask forming step) as shown in FIGS. 9B and 9C. It is noted thatin FIG. 9C as well as FIGS. 9E and 10B described below, only the “stepof forming and annealing a pattern of a channel precursor layer” isselected and imaged (plan view photographs) for clarity.

Subsequently, a pattern of the channel precursor layer 42 is formedusing the etching mask 80. Specifically, the etching mask 80 and thechannel precursor layer 42 are brought into contact with a commerciallyavailable etching solution (Product name: “ITO-02,” Kanto Chemical Co.,Inc.) in which the oxide precursor layer can be dissolved (typically,immersed into the etching solution) (a contact step). As a result, aportion of the channel precursor layer 42 not protected by the patternof the etching mask 80 is etched and removed as shown in FIGS. 9D and9E. On the other hand, a portion of the channel precursor layer 42protected by the pattern of the etching mask 80 is not etched, andremains on the gate insulator 34.

Subsequently, a second preliminary annealing step (a drying step) isperformed at a predetermined temperature (the first temperature) inorder to decompose the etching mask 80 (in particular, aliphaticpolycarbonate) and aliphatic polycarbonate in the channel precursorlayer 42 with which the pattern has been formed.

In the second preliminary annealing step according to the presentembodiment, heating is performed at a temperature where 90 wt % or moreof aliphatic polycarbonate is decomposed. Through the second preliminaryannealing step and the main annealing (the annealing step) describedbelow, the etching mask 80 itself and impurities contained in the finalchannel precursor layer 42 such as, in particular, carbonous impuritiesoriginated from aliphatic polycarbonate can be mostly eliminated. Inorder to reliably reduce residual impurities in the channel 44 such asin particular carbonous impurities originated from aliphaticpolycarbonate, the first temperature is preferably such that 95 wt % ormore of aliphatic polycarbonate is preferably decomposed at thattemperature, and more preferably such that 99 wt % or more of aliphaticpolycarbonate is decomposed at that temperature.

The second preliminary annealing step is not limited to include dryingat normal temperature under normal pressure. For example, the secondpreliminary annealing step can include treatment such as heating ordecompressing such as drying by heating, decompression drying, ordecompression drying by heating, as long as the substrate, the gateinsulator, and the like are not adversely affected. The secondpreliminary annealing step can possibly influence the increase ordecrease in surface roughness of the oxide semiconductor layer. Becausebehavior during drying differs depending on the solvent, conditions suchas the temperature (first temperature) in the second preliminaryannealing step are set appropriately in accordance with the type of thesolvent.

The second preliminary annealing according to the present embodiment canbe exemplified by heating the channel precursor layer 42 at atemperature in the range of from 180° C. or higher to 300° C. or lowerfor a predetermined period (e.g. 30 minutes). The above preliminaryannealing is performed in an oxygen atmosphere or in the atmosphere(hereinafter, also collectively called an “oxygen-containingatmosphere”), for example. According to an adoptable aspect, the secondpreliminary annealing step is performed in a nitrogen atmosphere.

Subsequently, in the main annealing, i.e., “the annealing step,” theetching mask 80 and the channel precursor layer 42 are heated, forexample, under an oxygen-containing atmosphere for a predetermined timeperiod in a range of 180° C. or above, more preferably 250° C. or above,even more preferably 300° C. or above, and still more preferably 500° C.or above in terms of electrical properties. As a result, the channel 44as an oxide semiconductor layer is formed on the gate insulator 34 asshown in FIGS. 10A and 10B. Strikingly, the etching mask 80 is alsodecomposed and/or removed in the main annealing step (the heating stepin this embodiment). This means that in this embodiment, removal of anetching mask and formation of an oxide layer (the oxide-layer formingstep) can be achieved simultaneously. Therefore, use of the etching mask80 can significantly simplify the production process. It is noted thatthe final thickness of an oxide semiconductor layer after the mainannealing is typically 0.01 μm or more and 10 μm or less. In particular,it is worth noting that the resulting layer is resistant to cracks evenwhen it has a thickness as thin as about 0.01 μm (or 10 nm).

Here, the preset temperature in the above annealing step is selected sothat it corresponds to a temperature at which a ligand of an oxidesemiconductor is decomposed in the oxide semiconductor forming process,and a metal thereof is allowed to bind to oxygen, and further so that itis equal to or above a temperature where an exothermic peak is observedin differential thermal analysis (DTA) as described above (the secondtemperature). In the above annealing step, aliphatic polycarbonate, adispersant, and an organic solvent in the channel precursor layer 42 areto be reliably decomposed and/or removed. It is noted that in apreferred aspect, the second temperature is higher than the firsttemperature by 10° C. or more in order to reliably reduce residualimpurities such as carbonous impurities in the oxide semiconductor layerafter the main annealing. In addition, when the second temperature ishigher than the first temperature by 50° C. or more, such residualimpurities can be reduced even more reliably. In the most preferredaspect, the second temperature is higher than the first temperature by100° C. or more in view of the controllability of the thickness and/orthe thinning of the final oxide semiconductor layer as well as reductionof residual impurities. Nonetheless, there is no particular limitationfor the maximum difference between the second temperature and the firsttemperature.

Our analysis suggests that aliphatic polycarbonate is mostly decomposedwhen heated to the first temperature, and thus in the subsequentannealing step (the main annealing) at the second temperature, almost nodecomposition reaction of aliphatic polycarbonate takes place, and areaction of bonding a metal with oxygen takes place almost exclusively.That is, ideally, two different functions of the first temperature andthe second temperature can allow cracks to be less generated even in thecase of a very thin layer as described above.

Meanwhile, the channel 44 protected by the etching mask 80 was found toshow a rectification behavior as a semiconductor when etching treatmentwas performed using the etching mask 80 according to the presentembodiment. Interestingly, a channel protected by using a commerciallyavailable resist as an etching mask was, however, found to show norectification behavior under some circumstances even though theconditions were the same except for the material of the etching mask.

FIG. 13 is a graph showing the rectification behavior of the channel 44which is a part of the thin film transistor 100 according to the presentembodiment. As shown in this graph, the rectification behavior as asemiconductor can be obtained when etching treatment was performed usingthe etching mask 80 according to the present embodiment. This can beattributed to reliable removal of the etching mask 80 in the heatingstep when forming a pattern of the oxide layer, or in other wordsreliable removal of residual impurities which may affect the electricalproperties of a semiconductor. Further, another striking advantage ofthe etching mask 80 according to the present embodiment is that use ofthe etching mask 80 according to the present embodiment can eliminateany effect on the so-called back channel region, which otherwise mayoccur upon detaching a resist. It is noted that the mixing ratio (by themass ratio) of indium and zinc used in the graph is adjusted to 1:1 byadjusting the molar ratio per kg of indium acetylacetonate and zincchloride. However, the mixing ratio of indium and zinc is not limited tothe above ratio. For example, the inventors of this application observedthe rectification behavior of the channel 44 even when the ratio ofindium to zinc was 2:1. Therefore, in a preferred aspect, the method ofmanufacturing a thin film transistor includes an oxide-layer formingstep of forming an oxide layer (the channel 44) protected by the etchingmask 80 in a way such that it makes contact with the gate electrode 24through the gate insulator 34, the oxide layer being formed via theheating step in the method of producing an oxide layer according to thepresent embodiment.

None of the first preliminary annealing step, the second preliminaryannealing step, and main annealing (the annealing step) described aboveis particularly limited in terms of its heating/annealing method. Theheating method can be exemplified by a conventional heating method withuse of a thermostat, an electric furnace, or the like. Particularly in acase where the substrate is less heat-resistant, it is preferred toadopt a method of heating/annealing only the oxide semiconductor layerby means of ultraviolet rays, electromagnetic waves, or a lamp forprevention of heat transfer to the substrate.

In the processes of removing the etching mask 80 and forming the channel44, aliphatic polycarbonate can contribute to reducing or eliminatingdecomposition products remaining in the oxide semiconductor layer afterthe annealing decomposition, and further contribute to forming minutepatterns of the oxide semiconductor layer. Therefore, in a preferredaspect of the present embodiment, aliphatic polycarbonate is used.

The studies conducted by the inventors of this application alsodemonstrated that in this embodiment, the final thickness of the channel44 was able to be controlled by varying the weight ratio of aliphaticpolycarbonate and a compound of a metal to be oxidized into an oxidesemiconductor, or varying the concentration of aliphatic polycarbonateor a compound of a metal. For example, our studies showed that thechannel 44 having a thickness ranging from 10 nm to 50 nm, which can beconsidered as a very thin layer, was able to be formed withoutgenerating cracks. It is noted that in addition to a thin layer asdescribed above, a layer having a thickness of 50 nm or more can beformed relatively easily by appropriately adjusting the thickness of thechannel precursor layer 42, the aforementioned weight ratio, and thelike. In general, a layer used for a channel has a thickness of 0.01 μm(i.e. 10 nm) or more and 1 μm or less. This means that the oxidesemiconductor precursor and the oxide semiconductor layer according tothe present embodiment, in which the final thickness of the channel 44can be controlled, are suitable as a material for manufacturing a thinfilm transistor.

In addition, when the oxide semiconductor precursor according to thepresent embodiment is used, the resulting oxide semiconductor precursorlayer, which may be considerably thick (e.g. 10 μm or more) at theinitial stage, can be made extremely thin (e.g. 10 nm to 100 nm) afterannealing because aliphatic polycarbonate and the like are reliablydecomposed in the subsequent annealing step. Strikingly, no crack isdeveloped, or development of cracks is reliably reduced even when theresulting layer is very thin as described above. This indicates that theoxide semiconductor precursor and the oxide semiconductor layeraccording to the present embodiment, which can be formed to have both asufficiently large initial thickness and a very thin final thickness,are very suitable for a low-energy production process or a knownimprinting process. Further, use of the oxide semiconductor layer, inwhich no crack is developed, or development of cracks is reliablyreduced even when it is very thin, will significantly improve thestability of the thin film transistor 100 according to the presentembodiment.

Further, in this embodiment, the electrical properties and stability ofthe oxide semiconductor layer which will form a channel can be improvedby appropriately selecting the type and combination of the oxidesemiconductors, and adjusting the mixing ratio of the oxidesemiconductor(s) and aliphatic polycarbonate.

(4) Formation of Source Electrode and Drain Electrode

Subsequently, a layer of the etching mask 80 according to the presentembodiment patterned on the channel 44 was further formed in a waysimilar as in the method of forming the channel 44. Then, as shown inFIG. 14, an ITO layer 50 is formed on the channel 44 and the etchingmask 80 by the publicly known sputtering method. The target materialused in this embodiment includes an ITO containing, for example, 5 wt %of tin oxide (SnO₂), and is formed under conditions of room temperatureto 100° C. Once the etching mask 80 is then removed by heating to about250° C., a drain electrode 56 and a source electrode 58 made of the ITOlayer 50 is formed on the channel 44. As a result, the thin filmtransistor 100 is manufactured as shown in FIG. 15. It is noted that inanother conceivable aspect, a pattern of a publicly known resist layerformed by the conventional photolithographic method may also be used inplace of the etching mask 80.

Furthermore, in this embodiment, a method of forming a drain electrodepattern and a source electrode pattern using paste silver (Ag) or pasteITO (indium tin oxide) according to a printing method, for example,instead of the above drain electrode 56 and source electrode 58, is anadoptable aspect. Moreover, a gold (Au) or aluminum (Al) drain electrodepattern and a gold (Au) or aluminum (Al) source electrode pattern formedby a known evaporation method may also be employed instead of the drainelectrode 56 and the source electrode 58.

Modification Example (1) of Second Embodiment

The thin film transistor according to this embodiment is similar to thethin film transistor 100 according to the second embodiment in terms ofthe production process and configuration thereof except that etchingtreatment was performed using the etching mask 80 after the step (themain annealing) of annealing a channel according to the secondembodiment. Therefore, descriptions common with the first or secondembodiment will be omitted.

In this embodiment, a step of forming a pattern of the channel 44 isperformed after forming the channel 44 as an oxide semiconductor layer.Therefore, in this modification example (1), etching of the oxideprecursor layer to be oxidized into an oxide layer as performed in thesecond embodiment is not performed.

FIGS. 16A and 16B are each a schematic sectional view of a process inthe method of producing a thin film transistor as an example of asemiconductor element. It is noted that the structure shown in FIG. 9Aaccording to the second embodiment has been formed before the step shownin FIG. 16A. Further, the structure shown in FIG. 10A according to thesecond embodiment will be formed after the step shown in FIG. 16B.

In a conceivable aspect, the channel 44 after the main annealing isformed before performing etching treatment as shown in this modificationexample (1). As shown in FIG. 16A, a pattern of the etching mask 80 asdescribed in the second embodiment is formed on the channel 44. Then,the etching mask 80 and the channel 44 are exposed to a solution fordissolving the channel 44 as an oxide layer (typically immersed intothis etching solution), and then heated to or above a temperature atwhich the etching mask 80 is decomposed. As a result, the channel 44 onwhich a pattern is formed can be obtained as shown in FIG. 16B. It isnoted that in this modification example (1), a portion of the oxidelayer (the channel 44) protected by the etching mask 80 (morespecifically, a pattern of the etching mask 80) can also show arectification behavior as a semiconductor.

Modification Example (2) of Second Embodiment

Further, in the second embodiment or in the modification example (1) ofthe second embodiment, a pattern of the channel 44 is formed via thecontact step of allowing contact with a predetermined etching solutionas a solution for dissolving an oxide precursor layer or a solution fordissolving an oxide layer, but in the second embodiment, it is notlimited to a treatment in which the etching solution is used. Forexample, in a conceivable aspect, an exposure step of allowing exposureto a publicly known gaseous plasma for etching the channel precursorlayer 42 as an oxide semiconductor precursor layer according to thesecond embodiment can be used to etch a region of the channel precursorlayer 42 which is not protected by the etching mask 80. As a result, aregion of the channel precursor layer 42 which has not been exposed tothe plasma will serve as the channel 44 after the subsequent heatingstep. In addition, the etching mask 80 can be decomposed and/oreliminated in the heating step. As a result, decomposition and/orremoval of the etching mask 80 can be achieved in one treatment step(the heating step) along with formulation of the channel 44. Further,the modification example (2) can also be applied to the examplesdescribed in the modification example (1) of the second embodiment.Specifically, in another conceivable aspect, the step of forming apattern of the channel 44 via the exposure step of exposure to plasmamay be performed after forming the channel 44 as an oxide semiconductorlayer. In all of the aforementioned cases, the channel 44 on which apattern is formed can show a rectification behavior as a semiconductor.

Modification Example (3) of Second Embodiment

In the second embodiment, the etching mask 80 formed by the screenprinting method is used to form a pattern of the channel 44. In anotherconceivable aspect, the etching mask 80 formed by the screen printingmethod may be used to form a pattern of other layers (for example, anoxide conductive material layer or an oxide insulator layer). Even whenthe etching mask 80 is used for the aforementioned other layers, theresulting oxide conductive material layer on which a pattern is formedcan show electric conductivity, and the resulting oxide insulator layeron which a pattern is formed can show insulation performance.

Alternative Embodiments (1)

In each of the above embodiments, an oxide precursor layer or an oxidelayer is a target of the etching treatment, but each of the aboveembodiments is not limited to these examples. For example, the etchingmask used in each of the above embodiments can be used to form a patternof a metal by etching a portion of that metal.

FIG. 17A is a plan view photograph showing the result of etchingtreatment of a chromium (Cr) layer using the etching mask according tothe first embodiment. Further, FIG. 17B is a plan view photographshowing the result of etching treatment of a chromium (Cr) layer usingan etching mask (a known resist mask) as Comparative Example of FIG.17A.

As shown in FIG. 17A, residues of the etching mask were not visuallyobserved when the etching treatment of a chromium (Cr) layer wasperformed using the etching mask according to the first embodiment.However, when the etching treatment of a chromium (Cr) layer wasperformed using the resist mask according to Comparative Example, manyresidues of the resist mask were visually observed as shown in FIG. 17B.It is noted that this etching treatment was found not to substantiallyalter the electrical properties (electric conductivity) as a metal.Further, even when other metals are etched, similar effects as the aboveare observed. Therefore, use of the etching mask according to the firstembodiment enables the etching mask after etching treatment to bereliably removed even when the target of etching treatment is a metal.

Alternative Embodiments (2)

Further, the etching mask used in each of the above embodiments can beused, for example, to form a pattern of an insulator by etching aportion of that insulator. FIG. 18 shows a plan view photograph showingthe result of etching treatment of a silicon dioxide layer (SiO₂) usingthe etching mask according to the first embodiment.

As shown in FIG. 18, when the etching treatment of the silicon dioxide(SiO₂) layer was performed using the etching mask according to the firstembodiment, no residue of the etching mask was visually observed. It isnoted that this etching treatment was found not to substantially alterthe electrical properties (insulation performance) of the silicondioxide (SiO₂) layer. Further, even when other insulators are etched,similar effects as the above are observed. Therefore, use of the etchingmask according to the first embodiment enables the etching mask afteretching treatment to be reliably removed even when the target of etchingtreatment is an insulator.

Alternative Embodiments (3)

Meanwhile, in the second embodiment and the modification examplesthereof, a thin film transistor having a so-called inverted-staggeredstructure is described, but each of the above embodiments is not limitedto this structure. For example, in addition to a thin film transistorhaving the staggered structure, a thin film transistor having theso-called planar structure in which a source electrode, a drainelectrode, and a channel are disposed on the same plane can also showsimilar effects as does each of the above embodiments. Further, inanother conceivable embodiment, the channel (that is, the oxidesemiconductor layer) in each of the above embodiments may be formed on abase material or a substrate.

As described above, the above embodiments and experimental examples havebeen disclosed not for limiting the present invention but for describingthese embodiments and experimental examples. Furthermore, modificationexamples made within the scope of the present invention, inclusive ofother combinations of the embodiments, will also be included in thescope of the patent claims.

INDUSTRIAL APPLICABILITY

The present invention can be applied broadly to the fields of electronicdevices including a mobile terminal, an information appliance, a sensor,or any other publicly known electric appliance including varioussemiconductor elements, MEMS (Micro Electro Mechanical Systems) or NEMS(Nano Electro Mechanical Systems), and medical devices, for example.

1. An etching mask for screen printing, comprising aliphaticpolycarbonate.
 2. The etching mask according to claim 1, wherein thealiphatic polycarbonate is substantially eliminated or removed byheating at 180° C. or more.
 3. The etching mask according to claim 1,formed from an etching mask precursor, wherein in the etching maskprecursor the percentage of the aliphatic polycarbonate having amolecular weight of 6000 or more and 400000 or less is 80% by mass ormore relative to the entire aliphatic polycarbonate.
 4. The etching maskaccording to claim 1, formed from an etching mask precursor, wherein inthe etching mask precursor the aliphatic polycarbonate has a value ofL/(D×v×η) of 0.25 mm⁻¹Pa⁻¹ or more in which a cylindrical bar made ofpolytetrafluoroethylene and having a diameter “D” is dipped in acollection of the aliphatic polycarbonate having a zero-shear viscosityη as measured using a rheometer (Model AR-2000EX, TA Instruments), andthe length “L” of a strand withdrawn from an outermost surface of thecollection of the aliphatic polycarbonate is then measured when thecylindrical bar is pulled upwardly at a velocity “v”.
 5. The etchingmask according to claim 3, wherein the contact angle of the etching maskprecursor to a base material is 30° or more and 36° or less at 30seconds after the etching mask precursor is placed on the base material,or the contact angle of the etching mask to the base material is 26° ormore and 32° or less at 120 seconds after the etching mask precursor isplaced on the base material.
 6. The etching mask according to claim 1,wherein a target of etching is an oxide layer, and the etching mask isfor forming a pattern on the oxide layer, and provides a portion of theoxide layer which shows a rectification behavior, the portion of theoxide layer having had been protected by the pattern, in a case wherethe oxide layer is obtained by bringing the etching mask into contactwith a solution for dissolving the oxide layer and then heating theetching mask to or above a temperature at which the etching mask isdecomposed.
 7. The etching mask according to claim 1, wherein the targetof etching is an oxide precursor layer to be oxidized into an oxidelayer, and the etching mask is for forming a pattern on the oxideprecursor layer, and provides a portion of the oxide layer which shows arectification behavior, the portion of the oxide layer having had beenprotected by the pattern, in a case where the oxide layer is obtained bybringing the etching mask into contact with a solution for dissolvingthe oxide precursor layer and then heating the etching mask to or abovea temperature at which the oxide layer is formed.
 8. The etching maskaccording to claim 1, wherein the target of etching is an oxide layer,and the etching mask is for forming a pattern on the oxide layer, andprovides a portion of the oxide layer which shows a rectificationbehavior, the portion of the oxide layer having had been protected bythe pattern, in a case where the oxide layer is obtained by exposing theetching mask to a plasma for etching the oxide layer and then heatingthe etching mask to or above a temperature at which the etching mask isdecomposed.
 9. The etching mask according to claim 1, wherein the targetof etching is an oxide precursor layer to be oxidized into an oxidelayer, and the etching mask is for forming a pattern on the oxideprecursor layer, and provides a portion of the oxide layer which shows arectification behavior, the portion of the oxide layer having had beenprotected by the pattern, in a case where the oxide layer is obtained byexposing the etching mask to a plasma for etching the oxide precursorlayer and then heating the etching mask to or above a temperature atwhich the oxide layer is formed.
 10. The etching mask according to claim1, wherein the aliphatic polycarbonate is at least one selected from thegroup consisting of polyethylene carbonate and polypropylene carbonate.11. An etching mask precursor for screen printing, comprising aliphaticpolycarbonate.
 12. The etching mask precursor for screen printingaccording to claim 11, wherein the aliphatic polycarbonate issubstantially eliminated or removed by heating at 180° C. or more. 13.The etching mask precursor for screen printing according to claim 11,wherein the percentage of the aliphatic polycarbonate having a molecularweight of 6000 or more and 400000 or less is 80% by mass or morerelative to the entire aliphatic polycarbonate.
 14. The etching maskprecursor for screen printing according to claim 11, wherein thealiphatic polycarbonate has a value of L/(D×v×η) of 0.25 mm⁻¹Pa⁻¹ ormore in which a cylindrical bar made of polytetrafluoroethylene andhaving a diameter “D” is dipped in a collection of the aliphaticpolycarbonate having a zero-shear viscosity η as measured using arheometer (Model AR-2000EX, TA Instruments), and the length “L” of astrand withdrawn from an outermost surface of the collection of thealiphatic polycarbonate is then measured when the cylindrical bar ispulled upwardly at a velocity “v”.
 15. A method of producing an oxidelayer, the method comprising an etching-mask forming step of forming apattern of an etching mask including aliphatic polycarbonate on theoxide layer by the screen printing method, a contact step of, after theetching-mask forming step, contacting the oxide layer with a solutionfor dissolving a portion of the oxide layer not protected by the etchingmask, and a heating step of, after the contact step, heating the oxidelayer and the etching mask to or above a temperature at which theetching mask is decomposed.
 16. A method of producing an oxide layer,the method comprising an etching-mask forming step of forming a patternof an etching mask including aliphatic polycarbonate on an oxideprecursor layer to be oxidized into the oxide layer by the screenprinting method, a contact step of, after the etching-mask forming step,contacting the oxide precursor layer with a solution for dissolving aportion of the oxide precursor layer not protected by the etching mask,and a heating step of, after the contact step, heating the oxideprecursor layer and the etching mask to or above a temperature at whichthe oxide layer is formed.
 17. A method of producing an oxide layer, themethod comprising an etching-mask forming step of forming a pattern ofan etching mask including aliphatic polycarbonate on the oxide layer bythe screen printing method, an exposure step of, after the etching-maskforming step, exposing the oxide layer to a plasma for etching a portionof the oxide layer not protected by the etching mask, and a heating stepof, after the exposure step, heating the oxide layer and the etchingmask to or above a temperature at which the etching mask is decomposed.18. A method of producing an oxide layer, the method comprising anetching-mask forming step of forming a pattern of an etching maskincluding aliphatic polycarbonate on an oxide precursor layer to beoxidized into the oxide layer by the screen printing method, an exposurestep of, after the etching-mask forming step, exposing the oxideprecursor layer to a plasma for etching a portion of the oxide precursorlayer not protected by the etching mask, and a heating step of, afterthe exposure step, heating the oxide precursor layer and the etchingmask to or above a temperature at which the oxide layer is formed. 19.The method of producing an oxide layer according to claim 15, whereinthe heating step comprises heating to or above a temperature at whichorganic substances are decomposed while applying ultraviolet rays to theoxide precursor layer and/or the etching mask.
 20. A method ofmanufacturing a thin film transistor, the method comprising an oxidelayer forming step of forming the portion of the oxide layer which hadbeen protected by the etching mask formed via the heating step in themethod of producing an oxide layer according to claim 15 so that theportion of the oxide layer makes contact with a gate electrode through agate insulator.
 21. The etching mask according to claim 4, wherein thecontact angle of the etching mask precursor to a base material is 30° ormore and 36° or less at 30 seconds after the etching mask precursor isplaced on the base material, or the contact angle of the etching mask tothe base material is 26° or more and 32° or less at 120 seconds afterthe etching mask precursor is placed on the base material.
 22. Themethod of producing an oxide layer according to claim 16, wherein theheating step comprises heating to or above a temperature at whichorganic substances are decomposed while applying ultraviolet rays to theoxide precursor layer and/or the etching mask.
 23. The method ofproducing an oxide layer according to claim 17, wherein the heating stepcomprises heating to or above a temperature at which organic substancesare decomposed while applying ultraviolet rays to the oxide precursorlayer and/or the etching mask.
 24. The method of producing an oxidelayer according to claim 18, wherein the heating step comprises heatingto or above a temperature at which organic substances are decomposedwhile applying ultraviolet rays to the oxide precursor layer and/or theetching mask.
 25. A method of manufacturing a thin film transistor, themethod comprising an oxide layer forming step of forming the portion ofthe oxide layer which had been protected by the etching mask formed viathe heating step in the method of producing an oxide layer according toclaim 16 so that the portion of the oxide layer makes contact with agate electrode through a gate insulator.
 26. A method of manufacturing athin film transistor, the method comprising an oxide layer forming stepof forming the portion of the oxide layer which had been protected bythe etching mask formed via the heating step in the method of producingan oxide layer according to claim 17 so that the portion of the oxidelayer makes contact with a gate electrode through a gate insulator. 27.A method of manufacturing a thin film transistor, the method comprisingan oxide layer forming step of forming the portion of the oxide layerwhich had been protected by the etching mask formed via the heating stepin the method of producing an oxide layer according to claim 18 so thatthe portion of the oxide layer makes contact with a gate electrodethrough a gate insulator.